This invention relates to a subsampling or decimation filter for subsampling a filter input sequence of oversampled data into a filter output sequence of subsampled data. Such a subsampling filter is used as a digital signal processing device in converting into a device output sequence of a low sampling rate a device input sequence of data which are obtained by subjecting an analog signal to one-bit encoding at a high sampling rate by, for example, a delta-sigma modulator or to oversampling analog-to-digital conversion at an oversampling ratio by an oversampling analog-to-digital converter.
An oversampled data sequence generally includes much quantization noise in higher frequencies. For signal processing, transmission, and accumulation, the oversampled data sequence must be converted into a converted sequence of data sampled at a low sampling rate and having a high resolution. The subsampling filter is used in such a case and is usually a finite impulse response (FIR) filter.
A compact subsampling filter of a novel circuit configuration is described in a paper contributed by Akira Yukawa, the present inventor, and two others to the ICASSP (IEEE International Conference on Acoustics, Speech, and Signal Processing) Proceedings, March 1985, pages 1400 to 1403, under the title of "An Oversampling A-to-D Converter for VLSI Digital "Codec's". In the manner which will later be described more in detail, this subsampling filter comprises circuit elements which must be put into operation at an undesiredly high frequency. Furthermore, the subsampling filter is appreciably power consuming. It is possible to compose the subsampling filter with circuit elements which are operable at a lower frequency. This, however, results in an objectionably increased amount of hardware.